… prenatal drug exposure can be harmful to unborn babies. Previous research has shown infants exposed to cannabis in the womb are 50 percent more likely to have a lower birth weight. Now three new studies presented Tuesday at the Society for Neuroscience annual meeting here suggest prenatal cannabis exposure—at least in rodents—could have serious consequences for fetal brain development.

In one study researchers at Washington State University in Pullman showed rat pups born to mothers exposed to high amounts of cannabis vapor during pregnancy had trouble with cognitive flexibility. Twice a day the scientists filled the pregnant rats’ containers with marijuana vapor from an e-cigarette, elevating levels of the psychoactive chemical THC (tetrahydrocannabinol) in the rats’ blood to roughly the human equivalent of smoking a joint. After the pups grew up the researchers trained them on a task that measured their ability to think flexibly and learn new rules. The young rats first learned to follow a light cue to push one of two levers in order to receive a sugary treat. The next day, pushing only the left lever would deliver the reward, regardless of which side the light had been on.

The rats exposed to cannabis in utero learned the first rule (following the light cue) without a problem, but they took significantly longer to learn the new rule (pushing the left lever) than did rats not exposed to the drug. The cannabis-exposed rats also made many more mistakes on the second day.

In a similar study, scientists at Auburn University in Alabama found rats born to mothers that had been injected with a low, continuous dose of synthetic cannabis during pregnancy were significantly impaired on several different memory tasks involving mazes…“There was a gap in the acquisition of the memory and the consolidation of the memory.”

The young rats whose mothers were dosed with the drug also had abnormalities in the hippocampus, the brain’s primary memory center. Specifically, they had difficulty creating new connections between neurons—the basis for forming new memories. The researchers think the differences in the hippocampus stem from changes in levels of glutamate, the brain’s main excitatory neurochemical involved in learning and memory..